Download VIR Lab Name:

VIR Lab (1.5 days)
ep3/04
Name: ______________________
Partners: _______________________________________
Date: __________
Period: ________
The following symbols are used in circuit diagrams for a:
RESISTOR (in this lab it looks like a very small brown box),
BATTERY (in this lab we will use a power supply instead but the symbol remains the same),
AMMETER (for measuring the current ”I”). Ours is calibrated in MilliAmps.
VOLTMETER (for measuring the change in electric potential from one point to another).
Resistor
Ammeter
+ Battery -
A
Voltmeter
V
NOTE: RECORD , just below the resistor symbol above, all the numbers and symbols written
on the resistor. You will need them later.
Define: Current (I) in amperes or amps or milliamps = # of charges to pass a given point in
one second.
Define: Voltage (V) in volts = change in electric potential between two points. Remember that
the change in electrical potential (voltage drop or rise) does not depend on how you get from one
place to another but rather on where those two places are (and on what batteries or power
supplies you have).
Attach one of the red posts on an ammeter to one end of the resistor. Attach the other end of the
resistor to one clip on the Power Supply (PS). Attach the black post on the ammeter to the other
clip on the PS. Turn on the power supply and turn up the current a little. If the ammeter reads
less than zero, reverse the power supply connections. If it reads beyond the scale, change scales
or reduce the PS dial.
A
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1. You now have a simple series circuit consisting of an ammeter and a resistor, with a power
supply as the voltage source. If the ammeter is not in series with the resistor, but is parallel
instead, why would it not read the correct current for the resistor (the charges going through
the resistor)? Finish the lab before answering the question here.
Without disconnecting the existing circuit:
Attach the black post of the voltmeter (blue meter) to one end of the resistor (x) and one of the
red posts of the voltmeter to the other side of the resistor (y). If the voltmeter reads less than
zero, reverse the voltage clips. If the voltmeter reads greater than the largest number on the
scale, either change which red post you are using or change the power supply setting.
V
x
y
A
2. You now have a parallel arrangement with the voltmeter and the resistor. Most charges will
go through the resistor, as they did before you attached the voltmeter. However a few will
now go through the voltmeter instead. They all were originally at point x and came back
together at point y. Since the voltmeter only measures charges moving through itself, why
does putting the voltmeter in parallel with the resistor give you the correct voltage for the
resistor? Finish the lab before answering the question here.
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DATA
Measure at least five spread-out current-voltage data pairs (I,V) for the resistor by changing the
power supply setting and recording the meter readings. Select a power supply setting that gives
high scale readings on both voltmeter and ammeter and then go to lower values in roughly
equal jumps (don't get picky here) until you are near zero on both meters. Before taking data,
make sure your settings can satisfy this requirement. Remember to read the scale that
corresponds to the posts you are using on the meters and stay on the same scale throughout. Note
that your ammeter measures current in milliamps but you need it in amps. Convert.
DATA SET 1:
Record in a data table (I,V).
HERE
Before you plot graphs, do the experiment described on the next page.
Plot a graph of V on the vertical axis and I on the horizontal axis.
If fairly straight, find the slope. AND write the equation for your line below, complete with
units. For credit, follow the standards developed in physics for graphing (that INCLUDES
writing the equation on the graph! Always!). Before graphing, collect the next set of data.
Examine the information in your equation. Is the slope important? What does it represent?
Is the intercept important. What does it represent?
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DATA SET 2: Read before you start.
Turn off the power supply and replace the resistor with a light bulb in a socket. Before turning on
the power supply, turn the power supply voltage all the way down. Then turn on the power and
set the voltage so the bulb is only moderately bright. “White” bright is too much, you risk
burning out the bulb. NEVER GO HIGHER THAN THIS.
Change the red post on the ammeter to the right-most red post. Remember to read the scale that
corresponds to that post. Measure at least seven (7) spread-out I,V data pairs for the light bulb.
Be sure to get several data points when the bulb is dark, with at least one reading less than
15 milliamps, if possible. You need extra data points at low currents in order to properly see any
pattern.
Graph the data on the computer. You should be able to see the rough trend of the data. If there
are large gaps in the data, particularly in the lower current values, you may need to collect more
data. The purpose here is to decide whether the entire graph is one big straight line or whether the
slope changes as you get to lower and lower currents. This means you must pick your extra data
points strategically. Your graph must support your answer convincingly for me as well as for you.
Record data and plot a good graph similar to what you did for data set 1..
DATA SET 2:
Record in a data table (I,V) data pairs.
HERE
3. Is this graph linear over the entire range of currents and voltages? Yes or No ?
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